Diffusers find wide-spread application in many products made by the electronics industry, for example, calculators, cellular telephones, and stereos, just to name a few. Presently, manufacturing cost and degree of reflectivity determine the commercial success of a diffuser.
FIG. 5 shows a cross section of a conventional, so-called transflective diffuser 2. This diffuser 2 is a solid, generally rectangular, clear plastic block which has a cylindrical aperture 4 formed adjacent to a front face 6 and a cavity in the underside of the block defining a ramped surface 8 extending between the aperture 4 and a back face 10. A portion of the top surface 12 located over the aperture 4 has a reflective coating 13a to prevent light emitted by light sources 16 (FIG. 4) in aperture 4 from propagating through that portion of the top surface. The ramped surface 8 is also coated with a reflective coating 13b; this coating reflects emitted light through the top surface 12 not covered by coating 13a.
FIG. 4 shows a liquid crystal display assembly that employs the transflective diffuser 2 shown in FIG. 5. The diffuser 2 is disposed on the surface of a substrate 14 so that the aperture 4 receives a pair of incandescent bulbs 16 hand soldered to the substrate surface. A color filter 18 is positioned on the diffuser top surface 12, a liquid crystal display panel 20 rests on top of the filter 18, and a cover 22 overlies the liquid crystal display panel 20. Electrical interconnects 24 and 26, which bracket diffuser 42, stand on electrical contact areas 28 on the substrate surface and provide electrical interconnections between substrate 14 and registering contacts on the underside of the liquid crystal display panel 20. Barbed tabs 29 on the cover 22 can be snapped into corresponding slots 30 in the substrate 14 to mechanically secure the components of this assembly to the substrate 14.
The light from bulbs 16 is transmitted through transflective diffuser 2 to the reflective coating 13b on the ramped surface 8 where it is reflected up to the liquid crystal display panel 20. This reflected light provides background illumination for the liquid crystal display so that the human eye can read the alphanumeric characters on the display when there is little or no ambient light present. The coating 13a of transflective diffuser 2 prevents concentrated areas of reflected light, called "hot spots", from appearing in the background of the liquid crystal display. Such hot spots, being easily discernible by the human eye, would degrade the quality of the backlighting.
The coatings 13a and 13b of diffuser 2 have largely determined the efficiency of this diffuser. That is, the degree of reflectivity of the coating is directly dependent on the quality of the coatings. The more reflective the coatings, the greater the portion of the emitted light that the diffuser will reflect up to the display panel 20, rather than refract through the walls of the diffuser. Unfortunately, the cost of coating the surfaces of the diffuser is also directly proportional to the coating reflectivity. Consequently, manufacturers have compromised by painting coatings onto the surfaces 8 and 12 which have both moderate reflectivity and cost.
The illustrated display assembly is disadvantaged also because it requires relatively intense incandescent bulbs because of light losses incurred in the transflective diffuser. In accordance with present day practice, the bulbs are soldered manually to the substrate 14. Also, since such bulbs are clear, a color filter 18 must be included to produce the required backlighting color. Both of these factors add appreciably to the overall cost of the assembly.